This invention relates to a gas sensor of the sintered tin oxide (SnO.sub.2) type having improved stability with respect to the passage of time, and more particularly to a gas sensor improved not only in respect of stability with respect to the passage of time but in respect of poison resistance against organic silicone and the like.
Conventional sensors used for detecting combustible gases such as methane gas, liquefied propane gas, town gas, hydrogen and carbon monoxide include catalytic oxidation type sensors, the latter utilizing metal oxides such as tin oxide, ferric oxide, and the like. Tin oxide semiconductor type sensors generally comprise a pair of electrodes and a sintered piece provided therebetween and containing tin oxide as a main component, the sintered piece also containing an electric conductance improver such as SnCl.sub.2, a heat resisting insulator such as silica, alumina or silica-alumina, and in some cases a precious metal catalyzer such as platinum, palladium, rhodium or the like. When such a gas sensor contacts a combustible gas and the sintered semiconductor absorbs the gas, its electric conductance increases sharply. When the combustible gas is no longer present the sintered semiconductor desorbs the gas therefrom and restores the electric conductance to an original value. To carry out the adsorption and desorption quickly, the sintered semiconductor which is the gas sensing unit is heated directly or indirectly and, generally, is constantly maintained at 300.degree.-450.degree. C.
Since the gas sensor is in use constantly energized to heat the sintered semiconductor portion to 300.degree.-450.degree. C. as noted above, the continuing high temperature, especially at times of high humidity, brings about changes in a surfacial fine structure of the sintered semiconductor and an increase in the electrical conductance of the sintered semiconductor. Therefore, depending on the lapse of use period and on use surroundings, an alarm may be given even when gas concentration is below a predetermined value at which the gas sensor is set to give an alarm. This has been a great problem of conventional alarm devices.
In recent years organic silicone materials such as silicone putty, silicone rubber, silicone oil and so forth have become widely used in construction. However, such organic silicone materials contain volatile oligomer which vaporizes into the ambient air. This silicone vapor adheres to the gas sensor surface and causes a problem of catalytic poisoning which results in deterioration of the gas sensor with the passage of time. A town gas alarm comprising the sintered piece of tin oxide will, when organic silicone adheres thereto, have an increased sinsitivity to miscellaneous gases such as of alcohol and give false alarms. At integrated circuit manufacturing plants silicone putty is extensively used on walls of clean rooms to maintain airtightness. Some semiconductor type chlorine gas alarms installed in such places have nil sensitivity in one month and in the case of catalytic oxidation type gas alarms the sensitivity sometimes is lost in one week. In either of the above cases the cause is considered to be organic silicone molecules having strong adhering activity which cover the activity points on the surface of the gas sensor and thus prevent the gas from being adsorbed thereto.